Transistors: How Are They Made?

In my last article about CPUs which you can check out here, I briefly mentioned how CPUs are made as well as transistors which is the topic of today’s article.

A minor knowledge in physics is needed, although I’ll mention most of it while going through it. So as I mentioned in my last article, CPUs are made out thin layers (wafers) of Silicon, but I didn’t mention how or why. And the reason is: Semiconductors! This type of material has a very distinct property which is its conductibility, it sits right in-between insulators and conductors making their use cases immensely varied. First, let’s get to understand what transistors are: -Transistors are pieces of Silicon, and by the nature of semi conductors, it only conducts electricity at a certain voltage. And that’s the reason to why sometimes they act like insulators.

To make the Silicon a bit more conductive something called doping, of all things, which is the process of adding a foreign atom to the Silicon. There are two types of doping: N-Type (Negative Type) and P-Type (Positive Type) and these are used together to increase the current flow. N-Type is done by shooting individual atoms of Phosphorus at the Silicon surface so it bonds together (Replacing one of the 5 Silicon atoms that form the molecule). And since Phosphorus has 5 electrons as opposed to the Silicon’s 4, the substance will have one more additional electron for every 5 atoms. In order to make this technique even worthwhile there’s also P-Type doping which is the exact same as N-Type but this time using Boron instead of Phosphorus. Boron only has 3 electrons so that means that there’s an empty spot for an electron in the bond.

And those spots are meant to be filled with electrons from the N-Type when a certain voltage is applied to the Silicon, making it a lot more conductive but not taking away from it’s highly useful properties. Think of it like this: N-Type is Room 1 and P-Type is Room 2 and there is a door between them. Room 1 has 50 beds but 60 people want to sleep in it, which of course, they can’t. Meanwhile, Room 2 has 50 beds, but only 40 people are sleeping there. The 10 remaining people will enter Room 2 when the door is open, and the door is the voltage. And for the people who need a reminder or don’t know yet: Electricity is just super excited electrons that move super fast.

“But why doesn’t low voltage travel through these substances if they’re this conductive?” You might ask. And that’s because in the contact point where the N-Type and P-Type Silicon contact, some electrons from the N-Type move to the P-Type atoms creating a form of small insulation called “The Depletion Layer” where no electron can passively go in nor out. When the needed voltage is applied electrons overcome the barrier and start moving.

So in order to make transistors out of a Silicon layer there needs to be a process to “cut” gaps into the Silicon to isolate every single transistor, and this process is called “etching”. There are several processes or “Processes Flows”for various types of transistors (i.e MOSFET etc…) We’re going to look at the MOSFET design because it’s simple and popular and it works like shown in the following image:

– You start by heating a Silicon wafer that is exposed to Oxygen in order to form a very thin layer of Silicon-Oxide (SiO) also referred to as “Silicon Rust” on top of the Silicon to be used as the main insulator for this particular design (More on that later). Then a layer of a material called “Photoresist” is applied on top of the SiO, this material is sensitive to UV light and it etches away when exposed to it. then a mask is applied on top of the Photoresist so it only etches the parts needed. After exposing it to UV light you end up with holes, a Plasma cannon” is used to etch away the Silicon. In general the trick in etching or ion implantation is that you need to lay down a thin layer on top of the substrate that effectively blocks the passage of ions or etching plasma through it and you need basically like cookie-cutter or stencil holes in that layer. This is where lithography comes in. Basically you have some chemical, called a “resist” that has the special property that if it has been exposed to light it will undergo a chemical reaction making it susceptible to the etching gas but if it hasn’t it resists being etched (it can also be the opposite case, see “positive” vs. “negative” resists). The final step is to use a very fancy stencil of light, called a “mask” that only allows light to pass through certain patterns in the “mask” and then you beam it with light. The stencil or mask determines the shapes where the resist is exposed to light and when you etch only those areas get etched away, the remaining resist protects the stuff you don’t want impanted. But it can only go small so far before Quantum effects start to take place since we’re talking about things at a size smaller than the visible light’s wave-length!

Something like Quantum Tunneling appears which basically makes electrons able to jump from a doped part to another, it’s just that! Your electrons start teleporting!